Water recovery is an important process in spacecraft environments and some other closed atmospheres. Dry air streams are desirable or necessary for certain essential spacecraft processes like carbon dioxide removal. Some carbon dioxide removal systems (molecular sieve) cannot tolerate humidity in the incoming fluid stream. Others (amine sorbent) undesirably lose the humidity content of the fluid stream. On short spacecraft missions, for example, moisture recovery may not be critical and amine sorbents can function adequately. However, on longer missions, it may be necessary to recover moisture so that it can be reused. In these cases, water recovery is usually addressed prior to carbon dioxide removal. In current practice, water recovery from process air utilizes desiccant media that are cyclically loaded by removing water from a humid process stream and unloaded by heating to desorb the adsorbed water and return it to the dry exiting process stream.
Conventional water recovery systems require large amounts of thermal energy and/or large pieces of equipment. Due to water's high heat of vaporization, large amounts of heat are released as the water vapor is removed from the process air stream and must be removed from the adsorbing dessicant bed to maintain a temperature favorable for effective adsorption performance. Conversely, a large amount of heat energy must be added to supply the heat of vaporization for water vapor release from the desorbing dessicant bed to the exiting process air stream before it is returned to the cabin. This heat input requirement is compounded by the need to heat the desorbing dessicant to an elevated temperature (above the adsorbing bed temperature) to effectively drive off the adsorbed water vapor. In conventional systems, the heat transport away from the desorbing bed and into the desorbing bed is commonly provided by the process air stream. This requires large temperature changes in the air and in the dessicant material because the specific heat of the air stream is small in comparison to the heat of vaporization of the water vapor that it carries. Energy supply requirements may be reduced by the use of regenerative process air heat exchangers, but their utility is limited by the need to raise the desorbing bed exit air flow to a temperature well above the temperature of the air exiting the adsorbing dessicant bed. While these heat exchangers can offer energy consumption and design integration advantages, they are significant contributors to the total system pressure drop, mass and cost. The system mass can be reduced by cycling smaller desiccant beds more rapidly, but this increases the energy requirements to support the thermal swings of the beds increasing the size of air heaters and heat exchangers as well as system power input. Thus, current water removal systems are inherently large and require large amounts of energy to operate.